### University of Crete HEP Seminars

**
Higher form symmetries and superfluids
**

Speaker: | Diego Hofman |

Institution: | University of Amsterdam |

Time: | Monday 23 September 2019, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | I will describe superfluid hydrodynamics as the hydrodynamic theory of a system with an emergent anomalous higher-form symmetry. The higher-form charge counts the winding planes of the superfluid -- its constitutive relation replaces the Josephson relation of conventional superfluid hydrodynamics. This formulation puts all hydrodynamic equations on equal footing. The anomalous Ward identity can be used as an alternative starting point to prove the existence of a Goldstone boson, without reference to spontaneous symmetry breaking. This provides an alternative characterization of Landau phase transitions in terms of higher-form symmetries and their anomalies instead of how the symmetries are realized. This treatment is more general and, in particular, includes the case of BKT transitions. |

**
Dark matter search and gravitational wave detection using atom interferometry
**

Speaker: | Wolf von Klitzing |

Institution: | IESL-FORTH Crete |

Time: | Thursday 26 September 2019, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | In a recent White Paper (arXiv:1908.00802) we proposed a concept for a space experiment using cold atoms to search for ultra-light dark matter, and to detect gravitational waves in the frequency range between the most sensitive ranges of LISA and the terrestrial LIGO/Virgo/KAGRA/INDIGO experiments. This interdisciplinary experiment, called Atomic Experiment for Dark Matter and Gravity Exploration (AEDGE), will also complement other planned searches for dark matter, and exploit synergies with other gravitational wave detectors. We give examples of the extended range of sensitivity to ultra- light dark matter offered by AEDGE, and how its gravitational-wave measurements could explore the assembly of super-massive black holes, first-order phase transitions in the early universe and cosmic strings. AEDGE will be based upon technologies now being developed for terrestrial experiments using cold atoms (also on Crete), and will benefit from the space experience obtained with, e.g., LISA and cold atom experiments in microgravity. |

**
The Geometric Trinity of Gravity: the TEGR case
**

Speaker: | Konstantinos Dialektopoulos |

Institution: | Center for Gravitation and Cosmology, Yangzhou University |

Time: | Friday 27 September 2019, 16:15 |

Venue: | 2nd floor seminar room |

Abstract: | It is well established by now that General Relativity and the LCDM model are very successful in describing the gravitational interactions at all scales. However, the theory is plagued with some shortcomings, enabling scientists to pursue an alternative formulation of gravity. In this talk, I am going to review the three different formulations of gravity; one based on the curvature of spacetime (General Relativity), one based on its torsion (Teleparallel Equivalent of General Relativity, TEGR) and one based on its non-metricity (Symmetric Teleparallel of General Relativity, STEGR). I will try to focus on the TEGR, explaining some of its key features. |

**
Fine Tuning Problems from Cosmological Coleman-Weinberg Potentials
**

Speaker: | Richard Woodard and S.P. Miao |

Institution: | UF (USA) and NCKU (Taiwan) |

Time: | Monday 21 October 2019, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | Cosmological Coleman-Weinberg potentials are quantum corrections to the effective potential of the inflaton which result from coupling it to ordinary matter in order make re-heating efficient. These corrections are problematic for inflation because they are not Planck-suppressed and tend to make the potential too steep. Therefore they must be subtracted off by additions to the classical action. Unfortunately, they cannot be completely subtracted off. Although they go over to the Coleman-Weinberg form in the flat space limit, their actual form on de Sitter background is the 4th power of the Hubble parameter times a complicated function of the coupling constant times the ratio of the inflaton to the Hubble parameter. On a general inflationary background they are not even local functions of the geometry, whereas the allowable subtractions consist of algebraic functions of the inflaton and the Ricci scalar. We discuss the disturbing results from the best possible local subtraction schemes. This talk is based on arXiv:1506.07306, 1806.02533, 1908.03814 and 1908.05558. |

**
Comments on the black hole S-matrix
**

Speaker: | Panagiotis Betzios |

Institution: | UOC |

Time: | Tuesday 5 November 2019, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | I will review the shockwave construction of 't Hooft by which gravitational backreaction can lead to a unitary S-matrix for particles scattering on an eternal black hole background. In order to obtain a background with a single exterior, an antipodal (CPT) identification was recently proposed that could in principle give a resolution to the information paradox for single sided asymptotically flat black holes. An emphasis will be given in the various assumptions and caveats of this construction. I will then describe an explicit dynamical quantum mechanical system that results in the same S-matrix. If time permits I will conclude with some efforts towards understanding the experience of an infalling observer in this setup. |

**
The Skyrmion Paradox
**

Speaker: | Christos Panagopoulos |

Institution: | NTU Singapore |

Time: | Tuesday 12 November 2019, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | Magnetic skyrmions (or vortices) are spatially inhomogeneous spin textures localized in nanoscale cylindrical regions. Topological protection and small size make skyrmions especially attractive for the study of spin topology and technologies wherein information is carried by the electron spin further to, or instead of the electron charge. Despite achievements in the synthesis of materials where axisymmetric magnetic skyrmions can be stabilized and characterized, there is disproportionate progress in elucidating the basic properties. The talk will attempt to bridge this gap and deliver an intelligible guide on the physical principles governing these magnetic whirls. |

**
Bootstrapping three dimensional cubic theories
**

Speaker: | Stefanos Kousvos |

Institution: | UOC |

Time: | Thursday 14 November 2019, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | After a concise introduction to the conformal bootstrap in general dimensions, I will discuss recent results regarding theories with cubic global symmetry in three spatial dimensions. Under certain assumptions, we find an isolated region in parameter space, which given prior intuition with the numerical conformal bootstrap, hints towards the existence of a CFT in this region of parameter space. We study the consistency of this CFT using multiple correlator systems. We find critical exponents for the conjectured CFT which are in discrepancy with the epsilon expansion (but in agreement with experiments for structural phase transitions). The disagreement of critical exponents for structural phase transitions calculated in the epsilon expansion with those measured in experiments is something that was noticed since the 70s. Given time I will briefly mention a resolution to this apparent discrepancy proposed at that time. |

**
Violation of the horizon effect after a quantum quench in the sine-Gordon model
**

Speaker: | Spyros Sotiriadis |

Institution: | University of Ljubljana |

Time: | Tuesday 4 February 2020, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | The study of non-equilibrium dynamics in quantum field theories is of central importance for a wide range of physics areas, from high-energy and cosmology to condensed matter and atomic physics. One of the most characteristic features of quantum dynamics in relativistic field theories (and more generally systems characterised by a maximum speed of information propagation) is the "light-cone spreading" of correlations, also known as "horizon effect": starting from an initially short-range correlated state, measurements of two observers at distant points are expected to remain independent until their past light-cones start overlapping. Surprisingly, we find that in the presence of topological excitations, correlations can develop outside of the horizon, even between infinitely distant points. We demonstrate this effect in the sine-Gordon model, showing that it can be attributed to the non-local nature of its topological excitations and interpret it as dynamical emergence of entanglement at large distances. Our findings are derived in two independent ways: (i) a novel numerical technique for the simulation of quantum many-body dynamics in continuous models, and (ii) an exact analytical method based on the duality between sine-Gordon and massive Thirring model. References: Phys. Rev. Lett. 121, 110402 (2018) arXiv:1802.08696 arXiv:1906.02750 |

**
The Origin of the Six-Particle Amplitude at Finite Coupling
**

Speaker: | Georgios Papathanasiou |

Institution: | DESY |

Time: | Thursday 6 February 2020, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | Scattering amplitudes form a bridge connecting theoretical particle physics with the real world of collider experiments. Yet their computation by means of Feynman diagrams quickly becomes prohibitive, and is only valid at weak coupling. Focusing on the simplest interacting gauge theory, known as large-color maximally supersymmetric Yang - Mills, and exploiting its integrability, in this talk I present a finite-coupling expression for its six-particle amplitude in a particular kinematic limit: The origin in the space of its natural kinematic variables, where two-particle Mandelstam invariants become much smaller than three-particle ones. I also demonstrate that our expression is in perfect agreement with existing field- and string-theoretic predictions for its weak and strong coupling expansion, respectively. |

**
Emergent fields from Hidden sectors
**

Speaker: | Pascal Anastasopoulos |

Institution: | Vienna University |

Time: | Tuesday 11 February 2020, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | Hidden theories coupled to the SM may provide emergent fields, that are composites/bound-states of the hidden fields. This is motivated by paradigms emerging from the AdS/CFT correspondence but it is a more general phenomenon. We explore the general setup and we focus on axions, graviphotons (or dark photons) and neutrinos. |

**
Consistent truncations of supergravity and twisted 4d
Superconformal Field Theories
**

Speaker: | Chris Rosen |

Institution: | University of Barcelona |

Time: | Thursday 20 February 2020, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | Superconformal field theories (SCFTs) in four dimensions are a cornerstone of contemporary theoretical physics. In the last decade, interesting classes of such theories have been constructed by compactifying six dimensional SCFTs on a Riemann surface. These theories, as well as some of their less supersymmetric deformations, are notable in that they lack a Lagrangian description. A powerful method for exploring the physics of such theories is provided by the methods of gauge/gravity duality. We illustrate this approach by first reviewing how the four dimensional fixed point theories are described by solutions to eleven dimensional supergravity. We then describe new solutions of the 11d theory that holographically encode deformations of the 4d SCFTs. Our techniques allow one to study these deformations in a five dimensional gauged supergravity theory, which arises as a consistent truncation of the 11d theory and greatly simplifies calculations. |

**
Uplifting Runaways and the Tadpole Problem
**

Speaker: | Severin Lust |

Institution: | Ecole Polytechnique |

Time: | Tuesday 25 February 2020, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | I will discuss a mechanism which can lead to a possible instability of the KKLT construction for de Sitter vacua. The sphere at the tip of a warped deformed conifold throat can be destabilized by antibranes placed in the throat. Consequently, the stabilization of moduli should not be treated independently from the antibrane uplift in KKLT-like scenarios. A similar bound can be found for numerically constructed Klebanov-Strassler black holes. This conifold destabilization mechanism can be avoided by turning on a large amount of flux on the sphere, but tadpole cancelation constraints the hierarchy of scales in a type IIB flux compactification. Even though sufficiently large tadpole bounds can be realized in string theory, they always come at the expense of a large number of moduli. The stabilization of these moduli by fluxes contributes to the tadpole condition as well, reducing the maximal flux on the KS-throat. I will discuss this problem for the example of M-theory on K3 x K3. |

**
Supersymmetric phases of N = 4 SYM at large N
**

Speaker: | Alejandro Cabo-Bizet |

Institution: | KCL London |

Time: | Tuesday 03 March 2020, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | We show the existence of an infinite family of complex saddle-points at large N, for the matrix model of the superconformal index of SU(N) N = 4 super Yang-Mills theory on S3 x S1 with one chemical potential t. The saddle-point configurations are labelled by points (m,n) on the lattice Lambda t = Z t + Z with gcd(m, n) = 1. The eigenvalues at a given saddle are uniformly distributed along a string winding (m, n) times along the (A, B) cycles of the torus C/Lambda t . The action of the matrix model extended to the torus is closely related to the Bloch-Wigner elliptic dilogarithm, and its values at (m,n) saddles are determined by Fourier averages of the latter along directions of the torus. The actions of (0,1) and (1,0) agree with that of pure AdS5 and the Gutowski-Reall AdS5 black hole, respectively. The actions of the other saddles take a surprisingly simple form. Generically, they carry non vanishing entropy. The Gutowski-Reall black hole saddle dominates the canonical ensemble when t is close to the origin, and other saddles dominate when t approaches rational points. |

**
Under the spell of gauge theory
**

Speaker: | Elli Pomoni |

Institution: | DESY |

Time: | Thursday 12 March 2020, (Colloqium 17:00) |

Venue: | 3rd floor seminar room |

Abstract: | Gauge theories provide the theoretical framework we use to describe the world we live in. Even though it is the most successful physical theory that men ever invented, it is still not known how to use it in order to explain a plethora of important phenomena such as confinement. In this talk I will take you on a trip through the modern developments in theoretical physics aiming to break this impasse. I will show to you how, based on very simple principles like symmetry and duality, remarkable progress has been made. |

**
Type-B Anomaly Matching and the 6D (2,0) Theory
**

Speaker: | Elli Pomoni |

Institution: | DESY |

Time: | Tuesday 17 March 2020, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | In this talk, we will study type-B conformal anomalies associated with 1/2 -BPS Coulomb-branch operators in 4D N = 2 superconformal field theories. When the vacuum preserves the conformal symmetry these anomalies coincide with the two-point function coefficients in the Coulomb branch chiral ring. They are non-trivial functions of exactly-marginal couplings that can be obtained from the S^4 partition function. We will examine the fate of these anomalies in vacua of the Higgs-branch moduli space, where conformal symmetry is spontaneously broken. We argue non-perturbatively that these anomalies are covariantly constant on conformal manifolds. In some cases, this can be used to show that they match in the broken and unbroken phases. Thus, we will uncover a new class of data on the Higgs branch of 4D N = 2 conformal field theories that are exactly computable. An interesting application of this matching occurs in N = 2 circular quivers which deconstruct the 6D (2,0) theory on a torus. In that context, we argue that 4D supersymmetric localisation can be used to calculate non-trivial data involving 1/2 -BPS operators of the 6D theory as exact functions of the complex structure of the torus. |

**
How Cosmological Coleman-Weinberg Potentials Depend on the Geometry of Inflation
**

Speaker: | Richard Woodard |

Institution: | University of Florida |

Time: | Thursday 26 March 2020, 14:15 |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | Cosmological Coleman-Weinberg potentials are induced by coupling the inflaton of scalar-driven inflation to ordinary matter in order to facilitate reheating. These potentials are not Planck-suppressed, and are far too steep to be consistent with inflation. However, they cannot be subtracted with local, lower derivative counterterms because they depend on the geometry of inflation in a complicated way. Explicit results have previously only been obtained for de Sitter background, in which the Hubble parameter is strictly constant. In this talk I describe an approximate computation which gives the result for arbitrary Hubble parameter. This paper is based on arXiv:1506.07306, 1806.02533, 1908.03814 and 1908.05558. |

**
Supersymmetry anomalies
**

Speaker: | Ioannis Papadimitriou |

Institution: | KIAS |

Time: | Tuesday 31 March 2020, 14:15 |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | Supersymmetric quantum field theories with an anomalous flavor or R-symmetry have a 't Hooft anomaly in Q-supersymmetry, i.e. the supercurrent is not conserved. I will first derive this anomaly by coupling the theory to either background vector multiplets or background supergravity in four dimensions, and solving the corresponding Wess-Zumino consistency conditions. A second derivation will be given in terms of anomaly inflow from a suitable supersymmetric Chern-Simons theory in one dimension higher. Although the supersymmetry anomaly can often be viewed as consequence of the Wess-Zumino gauge for the background supermultiplets, this does not mean that it is unphysical. I will show that the auxiliary fields in the off-shell superspace multiplet play the role of symmetry compensators, simply repackaging the same physical information. In particular, the supersymmetry anomaly implies that the supersymmetry transformation of the supercurrent receives quantum corrections, which in turn leads to a central extension of the supersymmetry algebra on curved backgrounds that admit Killing spinors. I will conclude by discussing how this modified algebra affects the supersymmetric partition function. |

**
Black Holes as a Hologram
**

Speaker: | Ioannis Papadimitriou |

Institution: | KIAS |

Time: | Wednesday 1 April 2020, 14:00 (Colloqium) |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | Black holes constitute the ultimate testing ground of fundamental physics. A century after their discovery as solutions of Einstein's equations of gravity, they continue to challenge our deepest understanding of nature, as well as our experimental capabilities. It was not until the operation of the LIGO and Virgo gravitational wave detectors in the last few years that a direct observation of black holes became possible. A consistent mathematical framework for quantum gravity capable of addressing key questions such as the black hole information paradox and the microscopic origin of black hole entropy was developed also relatively recently, with the discovery of string theory and the AdS/CFT correspondence. I will describe black holes as thermodynamic systems and as renormalization group flows in the context of holographic dualities. This will lead us to a discussion of recent advances in the identification of the microscopic degrees of freedom responsible for the macroscopic entropy of supersymmetric and near extremal black holes in terms of strongly interacting quantum systems. |

**
A review on lambda-deformations I
**

Speaker: | Konstantinos Siampos |

Institution: | UOA |

Time: | Tuesday 07 April 2020, 14:15 |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | We will revisit the construction of the lambda-deformed sigma models initiated in 1312.4560. These models are classically integrable, interpolating between exact current algebra CFTs at level k and the non-Abelian T-duals of principal chiral models. They are invariant under a weak-strong duality type symmetry which involves the deformation parameter lambda and the level k. Employing gravitational techniques, conformal perturbation and the aforementioned symmetry, we find the beta-functions and the anomalous dimensions of currents and current bilinears at leading order in the 1/k expansion. Afterwards, we will evaluate the 2- and 3-point correlation functions of the currents, deducing the OPEs and the equal-time commutators/Poisson brackets. These assume the underlying Poisson structure of the lambda-deformed sigma-models. Then we will consider generalized effective actions interpolating between exact CFTs. Employing CFT techniques we show how to compute, in the context of lambda-deformed CFTs, the exact in the deformation parameters C-function, satisfying Zamolodchikov's c-theorem. Extending the above results at two-loop in the 1/k expansion uncovers an extension of the duality type symmetry. Finally, we will illustrate how such deformations give rise to background solutions of type-II supergravity. Related papers: hep-th/9307030, 1312.4560, 1404.3748, 1407.2840, 1509.02946, 1604.08212, 1612.05012, 1707.05149, 1805.03731, 1906.00984, 1910.01056, 1911.02027 |

**
A review on lambda-deformations II
**

Speaker: | Konstantinos Siampos |

Institution: | UOA |

Time: | Thursday 09 April 2020, 14:15 |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | We will revisit the construction of the lambda-deformed sigma models initiated in 1312.4560. These models are classically integrable, interpolating between exact current algebra CFTs at level k and the non-Abelian T-duals of principal chiral models. They are invariant under a weak-strong duality type symmetry which involves the deformation parameter lambda and the level k. Employing gravitational techniques, conformal perturbation and the aforementioned symmetry, we find the beta-functions and the anomalous dimensions of currents and current bilinears at leading order in the 1/k expansion. Afterwards, we will evaluate the 2- and 3-point correlation functions of the currents, deducing the OPEs and the equal-time commutators/Poisson brackets. These assume the underlying Poisson structure of the lambda-deformed sigma-models. Then we will consider generalized effective actions interpolating between exact CFTs. Employing CFT techniques we show how to compute, in the context of lambda-deformed CFTs, the exact in the deformation parameters C-function, satisfying Zamolodchikov's c-theorem. Extending the above results at two-loop in the 1/k expansion uncovers an extension of the duality type symmetry. Finally, we will illustrate how such deformations give rise to background solutions of type-II supergravity. Related papers: hep-th/9307030, 1312.4560, 1404.3748, 1407.2840, 1509.02946, 1604.08212, 1612.05012, 1707.05149, 1805.03731, 1906.00984, 1910.01056, 1911.02027 |

**
Homogeneous Holographic Viscoelastic Model, Who Are You ?
**

Speaker: | Matteo Baggioli |

Institution: | IFT, Madrid |

Time: | Friday 10 April 2020, 11:15 |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | Models with broken translational invariance have attracted a great deal of interest in the holographic community in recent years, especially in relation to their hydrodynamic description and their possible relevance for strange metal phenomenology. Particular emphasis has been given to the so-called homogeneous models, due to their appealing simplicity. Despite the sustained activity in the field, there still remain a number of open questions. In this talk, I will discuss three fundamental points: 1) What is the correct hydrodynamic description for these models ? 2) What are these models describing ? 3) What is the meaning of the universal phase relaxation found in the literature ? I will provide a concrete and definitive resolution to the first point and I will propose a possible physical interpretation which provides simple answers to the rest of the open issues. |

**
The black hole S-matrix and Chaos
**

Speaker: | Panos Betzios |

Institution: | UOC |

Time: | Tuesday 28 April 2019, 14:15 |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | I will review the shockwave construction of 't Hooft, which takes into account a strong gravitational backreaction effect near the black hole horizon. This can lead to a unitary S-matrix for particles scattering on an eternal black hole background. I will then describe an explicit dynamical quantum mechanical system that results in the same S-matrix, whose Hamiltonian is related to the Dilation operator. In order to obtain a background with a single exterior, a global antipodal (CPT) identification of the Kruskal manifold was proposed by 't Hooft, that could in principle give a resolution to the information paradox for single sided asymptotically flat black holes. I will discuss the various problems present in this proposal and motivate a local form of the identification in phase space that takes into account the symmetries of the Hamiltonian. This provides a natural discretisation of the spectrum, that is shown to coincide with the zeros of the Riemann zeta and Dirichlet beta functions. It strengthens the proposal that our quantum Hamiltonian captures the dynamics of a Schwarzschild black hole, given the rich chaotic spectrum upon discretisation. It also explains why the spectrum appears to be erratic despite the unitarity of the scattering matrix. |

**
Entanglement and Algebraic Transformations on Topological Quantum Field Theories
**

Speaker: | Matthew Buican |

Institution: | QMUL |

Time: | Thursday 30 April 2020, 14:15 |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | I will describe certain natural algebraic transformations that one can perform on 2+1 dimensional topological quantum field theories (TQFTs) like Chern-Simons theories. These transformations allow one to sensibly explore the space of 2+1 dimensional TQFTs. In order to give these transformations physical meaning, I will explain how they preserve certain measures of entanglement. Throughout, I will connect these results to other areas of interest in the world of TQFT. |

**
Strings at the horizon
**

Speaker: | Ulf Danielsson |

Institution: | Uppsala University |

Time: | Thursday 07 May 2020, 17:00 |

Venue: | 3d floor seminar room (Colloqium) |

Abstract: | The physics of horizons is one of the deepest problems in fundamental physics. In this talk we explore the mysteries associated with event horizons of black holes, as well as dark energy and the cosmic horizon. Has string theory provided solutions or is there still something missing? |

**
Vortex Lattices From Holography
**

Speaker: | Aristomenis Donos |

Institution: | Durham University |

Time: | Tuesday 12 May, 14:15 |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | Vortices are fascinating stable configurations of electric currents that occur in the broken phase of superconductors. Based on an effective field theory approach, Abrikosov argued that when the force between them is repulsive, stable periodic configurations can form. In my talk I will consider similar constructions for superfluids away from the critical temperature. Moreover, I will discuss how known field theories can exhibit phases which are not captured by Abrikosov's considerations. |

**
Anisotropic Holography
**

Speaker: | Dimitrios Giataganas |

Institution: | NKUA |

Time: | Tuesday 19 May, 14:15 |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | Strongly coupled anisotropic systems have significantly richer structure than the isotropic ones. This is especially evident in the transport coefficients and in the phase diagram of anisotropic theories. We discuss such holographic theories, we comment on their observables and demonstrate how the isotropic universal relations are violated in presence of anisotropies. We also propose c-function candidates for the anisotropic RG flows and show that their monotonicity is not any more guaranteed by the energy conditions but depends on the theory characteristics. |

**
Transport, Strong Coupling and Black Holes
**

Speaker: | Aristomenis Donos |

Institution: | Durham University |

Time: | Thursday 21 May, 17:15 |

Venue: | (webinar Colloqium) |

Abstract: | One of the most fascinating aspects of many body systems is the collective behaviour of their degrees of freedom under external sources. The question is not only of fundamental importance but also of practical and technological importance in condensed matter systems. Models of weakly coupled particles have been extremely successful in understanding universal properties of transport. In my talk I will explain how black holes can help us understand transport properties of strongly coupled systems and probe regimes which are intrinsically non-perturbative. |

**
The connection problem for AdS2
**

Speaker: | Achilleas Porfyriadis |

Institution: | Harvard and Black Hole Initiative |

Time: | Tuesday 26 May, 16:15 |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | I will talk about the problem of extending anti-de Sitter solutions away from the near-horizon region of (near-)extreme black holes and connecting them with solutions in the black holes far asymptotically flat region. In Part I of the talk I will present analytic solutions to the connection problem for coupled gravitational and electromagnetic perturbations of AdS2xS2 and near-AdS2xS2 in the extreme and near-extreme Reissner-Nordstrom black holes, respectively. In Part II of the talk I will discuss a calculation of backreaction in AdS2. In this calculation, an AdS2xS2 throat geometry reacts to a perturbation by a scalar field in such a way that, with leaky boundary conditions, it maintains its connection to the far asymptotically flat region of the black hole. |

**
Bootstrapping theories with O(m)xO(n) global symmetry in three dimensions
**

Speaker: | Stefanos Kousvos |

Institution: | UOC |

Time: | Thursday 28 May 2020, 14:15 |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | I will discuss recent work performed with Johan Henriksson and Andreas Stergiou studying O(m)xO(n) symmetric conformal field theories in three dimensions. Motivated by phase transitions in three-dimensional systems, as well as open theoretical questions, we will focus on the O(2)xO(n) case. We study different values of n, for n sufficiently large we find excellent agreement with Large n predictions for scaling dimensions of operators. For n=3 we find closed isolated regions (islands) in parameter space that do not correspond to the ordinary fixed points of the d=4-e expansion, we attribute these regions of parameter space to fixed points conjectured to arise due to resummations of perturbative beta functions. |

**
Dynamics of Fluids without Boost Symmetries
**

Speaker: | Jelle Hartong |

Institution: | University of Edinburgh |

Time: | Tuesday 2 June 2020, 13:15 |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | Standard textbook treatments of fluid dynamics assume the presence of a boost symmetry. This is either a Galilean boost symmetry or a Lorentzian one and leads to the Navier-Stokes equations or their relativistic counterpart. It is however not necessary to rely on a boost symmetry as one can treat velocity as the chemical potential associated with momentum conservation. In fact for fluids moving through a medium one expects boost symmetries to be generically broken. This includes the case of scale invariant Lifshitz fluids. In this talk I will present the general treatment of fluid dynamics without boost symmetries and I will show that there can be 10 dissipative and 6 non-dissipative transport coefficients (of which 2 are hydrostatic and 4 are not) for uncharged non-boost invariant fluid dynamics. Time permitting, I will discuss some general ideas regarding holographic realisations of non-boost invariant Lifshitz fluids. |

**
Strong Cosmic Censorship versus Λ
**

Speaker: | Mihalis Dafermos |

Institution: | Cambridge and Princeton University |

Time: | Tuesday 9 June 2020, 13:15 |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | The strong cosmic censorship conjecture is a fundamental open problem in classical general relativity, first put forth by Roger Penrose in the early 70s. This is essentially the question of whether general relativity is a deterministic theory. Perhaps the most exciting arena where the validity of the conjecture is challenged is the interior of rotating black holes, and there has been a lot of work in the past 50 years in identifying mechanisms ensuring that at least some formulation of the conjecture be true. It turns out that when a nonzero cosmological constant Λ is added to the Einstein equations, these underlying mechanisms change in an unexpected way, and the validity of the conjecture depends on a detailed understanding of subtle aspects of black hole scattering theory, surprisingly involving, in the case of negative Λ, some number theory. Does strong cosmic censorship survive the challenge of non-zero Λ? This talk will try to address this Question! |

**
The holographic CFT masquerade
**

Speaker: | Mark Van Raamsdonk |

Institution: | University of Southern California |

Time: | Tuesday 23 June 2020, 19:00 |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | We argue that given holographic CFT1 in some state with a dual spacetime geometry M, and given some other holographic CFT2, we can find states of CFT2 whose dual geometries closely approximate arbitrarily large causal patches of M, provided that CFT1 and CFT2 can be non-trivially coupled at an interface. Our CFT2 states are ``dressed up as'' states of CFT1: they are obtained from the original CFT1 state by a regularized quench operator defined using a Euclidean path-integral with an interface between CFT2 and CFT1. Our results are consistent with the idea that the precise microscopic degrees of freedom and Hamiltonian of a holographic CFT are only important in fixing the asymptotic behavior of a dual spacetime, while the interior spacetime of a region spacelike separated from a boundary time slice is determined by more universal properties (such as entanglement structure) of the quantum state at this time slice. Our picture requires that low-energy gravitational theories related to CFTs that can be non-trivially coupled at an interface are part of the same non-perturbative theory of quantum gravity. |

**
The instability of Anti-de Sitter spacetime
**

Speaker: | Georgios Moschidis |

Institution: | University of California, Berkeley |

Time: | Tuesday 30 June 2020, 13:15 |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | According to the AdS instability conjecture, there exist arbitrarily small perturbations to the initial data of AdS spacetime which, under evolution by the vacuum Einstein equations with reflecting boundary conditions on conformal infinity, lead to the formation of black holes after sufficiently long time. To this day, the question about the vacuum equations remains open (and poorly understood). However, during the last decade, a large number of numerical and heuristic works have been dedicated to addressing the conjecture in the presence of matter fields allowing spherically symmetric dynamics. In this talk, I will review the progress achieved so far in addressing the AdS instability conjecture and I will present a rigorous proof of the instability in the setting of the spherically symmetric Einstein--scalar field system. The proof will be based on the non-linear interactions of special configurations of matter beams. The possible extension of the main ideas to the vacuum case will also be discussed. |

**
Liouville theory and Matrix models: A Wheeler DeWitt perspective
**

Speaker: | Panagiotis Betzios |

Institution: | University of Crete |

Time: | Thursday 2 July 2020, 13:15 |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | We analyse the connections between the Wheeler DeWitt approach for two dimensional quantum gravity and holography, focusing mainly in the case of Liouville theory coupled to c = 1 matter. Our motivation is to understand whether some form of averaging is essential for the boundary theory, if we wish to describe the bulk quantum gravity path integral of this two dimensional example. The analysis hence, is in a spirit similar to the recent studies of Jackiw-Teitelboim (JT)-gravity. Macroscopic loop operators define the asymptotic region on which the holographic boundary dual is expected to reside. Matrix quantum mechanics (MQM) and the associated double scaled fermionic field theory on the contrary, is providing an explicit "unitary in superspace" description of the complete dynamics of such two dimensional universes with matter, including the effects of topology change. If we try to associate a Hilbert space to a single boundary dual, it seems that it cannot contain all the information present in the non-perturbative bulk quantum gravity path integral and MQM. |

**
TBA
**

Speaker: | Costas Bachas |

Institution: | ENS Paris |

Time: | Tuesday 14 July 2020, 13:00 |

Venue: | 2nd floor seminar room (webinar) |

Abstract: | TBA |

**
TBA
**

Speaker: | Nicholas Warner |

Institution: | University of Southern California |

Time: | Tuesday 29 September 2020, 14:15 |

Venue: | 2nd floor seminar room |

Abstract: | TBA |